One funny thing about this mentorship is that you can do everything in your mentor way and still not make it in life. There is something about life, it's not about bed of roses, success has nothing to even do if you work everyday, you can work every day and still not be successful if life want to be unfair to you. You can be at the top of your game and life can bring you down under some seconds and there is nothing you can do or your mentor can do at that point.
True that, one of the reason is that people tend to keep the most essential secret to their student. It is rare to find a mentor that will expose all his knowledge, most just give people a basic guide and let them figure out the rest. This is the reason why even if we have mentors, it is hard for us to reach what they had achieved unless they will really guide us step by step and tell us their secret formula to success.
Those people are not really mentors, they are just sharing knowledge to show off how much they know, a true mentor would give all the information they have to their students and not only that but will show a path to grow even bigger than them, so that their students could improve the field more than they ever did.
A fake mentor will teach only "some" stuff, to make sure that they are known as the person who knows more than anyone else, it's ego, and that isn't what being a mentor is about, you can't grow a field by keeping secrets, after you die, the field is no better than you found it and you have helped it none. A true mentor would work tirelessly to raise as many great students as they can, to make sure they leave a better world behind.